The present invention relates to the non-destructive testing of workpiece properties and, in particular, to eddy-current probes for the non-destructive testing of pipes and holes of a small diameter, such as testing the coating in the through-holes of printed circuit boards.
Normally holes of printed circuit boards vary in diameter from 0.5 to 2.0 mm, whereas thickness of coating in through-holes varies from 15 to 50 microns. The material of electroplating is usually copper. There is not much prior art for testing within apertures of such small dimensions.
It is known in the art to use an eddy-current probe for non-destructive testing of electrically conductive plating in holes of printed circuit boards. Prior art probes used an oblong cylindrical core featuring excitation and measuring coils enveloping said core, whose wires are stretched along the axis of said core (U.S. Pat. No. 4,072,895, ). The known method for manufacturing said probe consists of winding an insulated wire on the cylindrical body as its coil (see, for example Reference Manual Non-destructive Testing, edited by R. McMaster, tr. from English, part 2, Moscow, Energy Publishers, 1965).
In the prior art the conductors of the coils envelop the cylindrical core and intersect at the butts or ends of the core. Winding such conductors is a delicate job due to minute dimensions of the core (less than 1 mm). Manual winding is usually performed with the aid of a microscope and so electrical characteristics of the prior art probes vary considerably. Moreover, conductors of the probe coils may be easily damaged when the probe is inserted into a hole to be tested.
Also known in the art is an eddy-current probe which is not so easily damaged (see, for example, German Patent No. 3,050,497, 1982). This probe is intended for non-destructive testing of metal coatings in the pipes and holes in the workpieces, and comprises a carrying cylindrical core and a enveloping coil whose wires are stretched along the axis of the core. All the wires are secured on in the lateral surface of the core. Said coil carries a special coating to be protected from mechanical damage.
Moreover, said patent describes the method of manufacturing for such probes including manufacturing of the flat elastic substrate made of dielectric material with dimensions chosen according to the core size, forming the coil and securing of the formed coil on the surface of the core. The coil forming may be made through, for example, photo-lithography or laser processing of the pre-metallized surface.
This method gives some advantages: small dimensions, technologically-based design, high sensitivity, and a probe output which is invariant to changes in the radial and axial placement of the probe with respect to the axis of the hole under test. But during the application this probe has insufficient mechanical stability and may suffer from a short life time for the coil's protective coating. The low durability of this probe is the result of the high core length-to-diameter ratio. Since the probe's outside diameter is, at the limit, diameter defined according to the holes to be tested, the protective coating's enlargement results in the reducing of the carrying core's diameter, and so, diminishes the mechanical durability of the probe.
This problem becomes acute during the testing of coatings in through holes or vias in printed circuit boards. In such testing one encounters sharp edges within the through holes. This probe is liklely to encounter multiple mechanical influences and yet is limited in core size to approximately 0.5 mm.